Rotorcraft are now a well-established means of transportation, and generally offer a vertical takeoff and landing capability, making them particularly valuable for transport applications without access to lengthy runways. Helicopters are the predominant type of rotorcraft, and have widespread civil and military application. In contrast to fixed-wing aircraft, which use wings to generate sufficient lift to sustain flight, rotorcraft use spinning rotors to generate lift at least in rotor-borne flight.
Rotors comprise blades that can rotate in the air about an axis. As these blades rotate, blade sections encounter an air velocity which is the vector sum of rotorcraft motion, rotor rotation, and air inflow. As a blade section, generally of an airfoil shape, encounters this velocity it produces lift in a direction perpendicular to the velocity vector. Associated with the generation of this circulatory lift is the generation of shed and trailed vorticity into the air. Strong bundles of vorticity are trailed from the tips of the rotor blades, forming a rotor wake. In many flight conditions, the rotor wake can be visualized as a set of intermeshed helices that form from the rotor and gradually decay. This trailed vortex wake can have a strong effect on local rotor blade loads.
The edgewise advance ratio of a rotor is defined as rotor forward velocity divided by rotor tip speed, or μ=V cos(α)/(ΩR). For an ideally hovering rotor, the advance ratio is zero, and the rotor blades trail an un-skewed helical wake below the rotor. As the rotor begins forward motion and advance ratio increases, the wake skews backward. As rotor blades pass near or through trailed vorticity, there is a strong aerodynamic interaction, leading to changes in local blade loading. These changes can lead to reduced performance from increased induced losses, increased vibration, and increased noise. The classical problems of rotor aerodynamics are discussed in the 2006 book “Principles of Helicopter Aerodynamics, 2e” by J. Gordon Leishman.
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Modern prior art helicopters use specially designed rotor blades to help reduce the adverse effects of interaction between trailed vorticity and blade loading. Some helicopter blades use special tip shapes to modify the vortex trajectory or separate the trailed vorticity into multiple bundles. However, all of the prior art methods have limited effect because dramatic changes in tip geometry will create increased drag and adversely affect performance.
U.S. Pat. No. 5,199,851 to Perry, et al. describes helicopter rotor blades with a tip vane for reducing helicopter blade noise that covers the outer 4 to 8% of the blade and has a dihedral of about five degrees. U.S. Pat. No. 4,324,530 to Fradenburgh, et al. discloses a twisted helicopter blade with the outer 4% of the tip having an anhedral of twenty degrees for improving the rotor performance in hover. US Patent Application 2005/0265850 describes a helicopter rotor blade with a small protruding tip vane that can have an anhedral angle. Similarly, U.S. Pat. No. 6,142,738 to Toulmay describes a helicopter blade with a small tip winglet for reducing noise, the winglet having a leading edge sweep between twenty and thirty degrees.
What these prior art systems and methods have in common is that they all involve relatively minor adjustments to blade tip geometry, both in terms of the percent span affected, and in terms of the amount of anhedral or sweep. There is good reason for this because, for example, excessive anhedral will greatly increase rotor drag in forward flight, negating any benefit of increased hover performance and compromising the utility of the aircraft. Likewise, excessive rotor blade sweep can compromise the vibrational characteristic and aeroelastic stability of the rotor.
In the related field of tiltrotors, rotor blades are generally straight, and do not feature complex tip geometry. As examples of tiltrotors with straight blades, consider the prior art Bell™ V-22, XV-15, and BA-609 tiltrotor aircraft, or U.S. Pat. No. 6,607,161 to Krysinski, which all have straight, tapered blades without tip sweep, tip anhedral, or tip vanes. Because tiltrotors spend much of their flying time in airplane mode with the rotors operating as propellers, there is little need or motivation to create specialized blade tips to alleviate helicopter mode noise, vibration, and performance issues.
Similarly, airplane propellers benefit from tip sweep, but not anhedral or other more complex tip geometries because airplane propellers substantially do not engage in edgewise flight, but remain in predominantly axial flight throughout operation. For propellers, blade-vortex interaction or hover performance are of essentially no concern. U.S. Pat. No. 5,927,948 to Perry, et al. describes a propeller blade with a tip portion of enlarged chord and some sweep, but without any anhedral.
In general, tiltrotors have highly twisted blades and operate at higher thrust coefficients as compared with helicopters. The higher thrust coefficients tend to improve performance in airplane-mode axial flight at the expense of some hover performance. Because tiltrotor aircraft generally hover on takeoff and benefit from ample vertical takeoff capability, there is a continued need for improved tiltrotor hover performance.
Thus, there is still a need for advanced rotors for tiltrotors that can improve hover performance and maintain or improve axial forward flight performance.